This application claims the priority benefit of Japanese Patent Application No. 11-37368, filed Feb. 16, 1999, the entire subject matter of which is incorporated herein of reference.
1. Field of the Invention
The invention relates to a wire bonder which connects a pad on a semiconductor chip with a pad on a circuit board so as to make a looping with a bonding wire.
2. Description of the Related Art
A wire bonder of an ultrasonic thermocompression type includes a pipe-shaped capillary through which a gold bonding wire made of gold can be passed through, a cut damper and a torch rod. The cut damper has a pair of disk-shaped clamping electrodes made of cermet (Crxe2x80x94SiO), which are placed on parallel plates. The clamping electrodes can clamp the gold bonding wire and apply a fixed electric potential to it.
In a case where a tip of the gold bonding wire is to be connected to a pad on a circuit board, a gold ball should be formed at the tip as the tip exits the end of the capillary. Then, the gold ball is pressed by the tip of capillary against the pad on the circuit board, and is bonded to the pad using an ultrasonic vibration. In a case in which the gold bonding wire connected to the pad on the circuit board at its tip, is to be connected to a pad on a semiconductor chip, the capillary is moved to a position above the pad on the chip. Then, the gold bonding wire is pressed by the tip of capillary against the pad, and is bonded to the pad by applying ultrasonic vibration. After the connection has been completed, the gold bonding wire is clamped by the clamping electrodes of the cut clamper, and is pulled by the cut clamper. The gold bonding wire is cut by tension.
The gold ball is formed using the following method. The gold bonding wire is clamped by the clamping electrodes. Then, by applying an electrical voltage between the electrodes and the torch rod, a melted gold ball is formed by atmospheric discharge against the gold bonding wire as it exits the end of the capillary. Then, the melted gold ball is solidified by cooling.
The circuit board is heated when the gold bonding wire is bonded thereto. Therefore, the gold bonding wire which is pressed by the tip of the capillary is ultrasonically thermocompressed by applying ultrasonic vibration to the gold bonding wire.
However, because the face of the electrode for clamping the gold bonding wire is disk-shaped, if the gold bonding wire is clamped in a position tilted from the axis of disk-shaped electrode, the distance between the end of the electrode and the tip of the gold bonding wire is different from what is desired. Discharge voltage is varied with a parameter of the distance. This may cause the gold ball to be made in various sizes. Additionally, the face of the electrode is worn because of the repetition of the atmospheric discharge. This also causes the gold balls to vary in size. The strength of the gold ball when pressed on the pad depends on its size. Thus, as the size varies so does the bonding strength. Therefore, nonuniformity in gold ball size disadvantageously results in nonuniformity in bonding strength.
Further, the faces of the clamping electrodes should be precisely in parallel to clamp the gold bonding wire. As the result, as electrodes come out of parallel with use, the electrodes must be periodically changed.
It is an object of the invention to provide an improved wire bonder that corrects the problems mentioned above.
To achieve this object, a wire bonder according to the invention includes a cut damper having a first supporting member, a second supporting member which is opposite the first supporting member, the second supporting member being movable toward and away from said first supporting member, a first clamping electrode connected to the first supporting member, the first clamping electrode having a first rectangular main surface, and a second clamping electrode connected to the second supporting member, the second clamping electrode having a second rectangular main surface which faces the first rectangular main surface in parallel relation thereto.
Further, to achieve the object, the first clamping electrode and the second clamping electrode are rotatably connected to the first supporting member and the second supporting member, respectively. Also, the first and second clamping electrodes have even surfaces, and a heater is incorporated in one of the support members, or in both.
In addition, to further achieve the object, the cut damper is divided into two independent dampers having different functions. The first damper serves to apply an electric potential to the gold bonding wire, and the second damper serves to clamp the gold bonding wire in order to pull the gold bonding wire.